1. Field of the Invention
The present invention relates generally to power supplies, and more specifically to power supplies for charging capacitors.
2. Discussion of the Related Art
Capacitors are energy storage devices that are capable of storing energy, which may be very rapidly discharged and then subsequently recharged. Conventional capacitor charging circuits typically utilize a DC source, such as a battery, which is used to charge the capacitor. Typically, current is allowed to flow into a primary winding of the transformer to store energy, which is then discharged into the capacitor in the form of a charging current flowing through the secondary winding of the transformer into the capacitor to be charged. Such charging circuits oscillate between storing energy into the transformer (a charging cycle of the transformer) and then transferring this energy into the capacitor (a discharging cycle of the transformer) until it is charged to the desired level.
However, a completely discharged capacitor appears as a dead short, e.g., 0 volts at the output, which causes excessive current surges that may damage the charging circuit. These current inrushes occur for a relatively long time since as the voltage at the output is near zero, the transformer takes considerable time to discharge. Furthermore, the time period of the charging and discharging cycles are set. Thus, if the transformer is not allowed to sufficiently discharge, additional energy transferred into the transformer in successive charging cycles may saturate the transformer. In order to allow the transformer to sufficiently discharge during the initial current surges, a current limit path, e.g., a choke inductor or a resistor, is used to provide a path for the current such that the secondary of the transformer can be adequately discharged before the next charging cycle begins. Disadvantageously, the majority of the initial energy is not transferred to the capacitor. Once the voltage level of the capacitor reaches a point where it is safe for the entire discharging current to flow into the capacitor without saturating or damaging the charging circuit, the current limit path is removed.
The switching of the charging and discharging cycles can be variously controlled. For example, in many DC—DC converters in which a battery source is used to charge a capacitor, the switching is set at a fixed frequency. That is, after a predetermined time period, the charging cycle is switched to the discharging cycle, and then the switched back to the charging cycle.
In another example, such as described in U.S. Pat. No. 4,104,714, issued Aug. 1, 1978, a battery of a DC—DC Converter is coupled to a transformer to charge the capacitor for use in a gas ignition system or photographic flash, for example. The circuit is designed to regulate the battery current in order to obtain maximum power transfer from the battery through its useful life. The charging cycle is terminated or switched when the current flowing in the primary of the transformer reaches a predetermined level. The discharging cycle is terminated or switched based on when the current in the secondary drops to a zero level threshold (all of the energy has discharged) and when the battery voltage rises back to a threshold. Thus, there is a delay between the discharge cycle and the start of the charging cycle to allow the battery to return to a given voltage level so that it can maintain maximum power transfer from the battery. Therefore, the restarting of the charging cycle is dictated by the battery voltage level. This can lead to excessive charge up times while the battery recovers and in some cases, the battery may not be able to supply enough current in the charging cycle to reach the predetermined level.
In another example, such as described in U.S. Pat. No. 4,272,806 issued Jun. 9, 1981, a battery of a DC—DC Converter is coupled to a transformer to charge the capacitor for use in an electronic strobe flash unit. This circuit is designed to be an optimal compromise in battery performance and converter performance. The charging cycle is or terminated or switched when the voltage of the battery source decays to a predetermined level. The discharging cycle is terminated or switched when the current flowing into the secondary lowers to a predetermined level.